"What's the best way to determine true airspeed?" - Archer H.
“For planning purposes, the first place I would go would be your Pilot Operating Handbook and in a modern handbook you would look in Section #5 – Performance. Find the cruise performance charts that indicate true airspeed (as well as fuel consumption) at various altitudes, power settings and temperatures.
If you are dealing with an older airplane without a detailed POH, you can develop TAS via an E6B flight computer by inputting altitude, temperature, and indicated airspeed. You can also use this technique with a modern airplane to verify or fine tune the numbers found in the performance section of the POH.
Lastly, if your airplane has an airspeed indicator equipped with a true airspeed ring, you can input altitude and temperature and read true airspeed on your airspeed indicator.
Know your Pilot Operating Handbook and become familiar with the information available for your aircraft.”